1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine and, more particularly, to a spark plug for an internal combustion engine having a semi-surface discharge gap.
2. Description of the Related Art
Semi-surface discharge type spark plugs have been known as spark plugs for internal combustion engines having improved anti-fouling properties. Such a semi-surface discharge type spark plug (hereinafter also referred to as a semi-surface spark plug) is similar to a normal aerial discharge type spark plug in that it includes a center electrode, an insulator provided around the outer periphery of the electrode, a cylindrical metal shell provided around the outer periphery of the insulators and a ground electrode having a base end bonded to the leading end of the metal shell. The semi-surface spark plug is configured such that application of a voltage between the center electrode and the ground electrode forms a spark discharge path (spark discharge gap). The spark discharge path is constituted by an aerial discharge gap between a part of a leading end of the ground electrode and a part of a leading end of the insulator and a surface-creeping discharge path beginning at a part of the leading end of the insulator and a part of a region of the center electrode in the vicinity of the leading end surface of the insulator.
It is known that when a spark plug is operated over a prolonged period of time in a low temperature environment, the spark plug enters what is called a “smoldered” or “covered” state in which a leading end surface of the insulator is covered by a conductive fouling substance such as carbon. This results in a state called “fouling” that disables proper firing at the spark discharge gap. In this regard, a semi-surface spark plug as described above has improved anti-fouling properties as compared to an aerial discharge type spark plug. This is because fouling substances are burned off by spark discharge which takes place along the leading end surface of the insulator (for example, see JP-A-2003-22885).
3. Problems to be Solved by the Invention
The durability of a spark plug must be maintained by extracting sufficient heat from the ground electrode. In order to maintain the durability of the ground electrode at a predetermined level, a somewhat large size ground electrode is used in practice.
However, when attempting to satisfy the demand simply by using a large-sized ground electrode, a space for allowing the flame kernel to spread can be obstructed. Also, the beat of the flame is likely to be removed by the ground electrode, which can worsen the quenching effect. As a result, the ignition capability may be reduced.
Compared to an aerial discharge type spark plug, a semi-surface spark plug tends to exhibit a higher possibility of fuel bridging. This is because the ground electrode in such a structure necessarily faces the insulator over a large area. When the size of the ground electrode is further increased as described above, the risk of fuel bridging further increases.
JP-A-2003-22885 discloses a noble metal tip provided on a ground electrode. However, a reduction in ignition capability and the risk of fuel bridging still remains when the countermeasure is to simply provide a tip, although a certain level of durability can be achieved.